1. Field of the Invention
This invention relates to an interlayer connection structure between conductive circuit layers disposed at one and the other surface sides of a circuit board.
2. Brief Description of the Related Art
Recently, electronic instruments have developed remarkably and electronic instruments of high degree of precision have been used in various fields. With the progress of this development, circuit boards to be used in such electronic instruments are required to have a high performance and a high efficiency. At the same time, such electronic instruments are required to be inexpensive.
As a circuit board, a rigid multilayer circuit board, flexible multilayer circuit board and a rigid/flexible multilayer circuit board as a combination thereof are mostly used. In such a multilayer circuit board, an interlayer connection is required. In order words, conductive circuits of the multilayer circuit board are required to be electrically connected together at predetermined spots.
FIG. 1 shows a process for an interlayer connection by a through-hole plating method which is most generally practiced. According to this method, as shown in FIG. 1A, a double face copper clad lamination substrate composed by joining copper foils 1, 2 to opposite surfaces of an insulative sheet 3 is formed. Then, as shown in FIG. 1B, a part of this double face clad lamination substrate corresponding to a conductive circuit is drilled, for example, to form a hole therein. Thereafter, as shown in FIG. 1C, a copper plating 5 is applied to an inner wall surface of a drilled hole 5. At this time, the copper plating 5 is applied also to the surfaces of the copper foils, 1, 2 on the opposite surfaces of the insulative sheet 3. Then, as shown in FIG. ID, a pattern is formed on each copper foil 1, 2 by an ordinary photoetching process, so that conductive circuit layers 1', 2' are formed. This example is a method for manufacturing a double face circuit board The same is likewise applicable to a multilayer circuit board which is subjected to the drilling process and the copper plating process in which the above-mentioned drilling and copper plating processes are employed.
In such prior art devices as mentioned above, several thousands of holes free from burrs must be formed in the conductive circuit. Moreover, in some cases, a surface processing (smearing processing) must be performed with respect to the surface of the wall of each hole.
That is, because a large number of processing steps are required for the interlayer connection, those steps result in major factors for increasing costs. Moreover, because there is a provision of holes formed in the surface of the board, electrical parts can not be mounted thereon. This makes it difficult to mount such electrical parts in high density.
The present invention has been accomplished in view of the above-mentioned problems inherent in the prior art devices.